Small vehicles such as utility tractors utilize transmissions such as hydrostatic and clutch driven units. These transmissions rely upon hydraulic oil pumps to circulate the fluid from a reservoir located within the transmission case, through filtering elements to the pump and then back to the transmission and fluid reservoir. Typically the hydraulic fluid reservoirs are provided within the transmission case itself.
Since the hydraulic fluid becomes contaminated with both larger and smaller particles over time as it is used, the hydraulic fluid is passed through strainers to remove the larger contaminates and filters to remove the smaller contaminates.
In vehicles such as those used in the present application, the filters are typically designed to remove contaminates having sizes up to approximately five microns and larger, while the strainers are typically designed to remove contaminates having sizes in the 60 micron and larger range.
Present hydrostatic fluid systems on these vehicles provide for separate strainers and filters to be plumbed in series with the strainers provided upstream of the filter. A bypass valve is then located between the strainer and filter to direct the hydraulic fluid around the filter when the hydraulic fluid is cold, as will be the case during initial startup periods. During those times, the viscosity of the oil can be so high that the vacuum generated by the pump can cause the filter to collapse or be damaged and/or the pump to be damaged. Therefore, the bypass valve serves to direct the hydraulic fluid around the filter until the viscosity drops and the pump suction pressure drops to levels which are safe enough for the filter to handle.
Because the strainers and filters are plumbed in series, the hydraulic pump operates to pull fluid through the strainer at all times, even after the oil is warm and the viscosity is low. Accordingly, the hydraulic system loses some of its horsepower capability when it must constantly pull the fluid through the strainer. It would, therefore, be desirable to eliminate this parasitic horsepower loss caused by the need to always pull the hydraulic fluid through the strainer.
An additional problem presented in current hydraulic fluid systems on present vehicles relates to the plumbing arrangement. Since the filter elements need to be periodically changed when they become full of contaminates, and since the strainer elements need to be periodically cleaned or changed as they fill up, both elements are housed in plumbing outside of and adjacent to the transmission case to provide easier access to them. Locating them outside of and/or adjacent to the transmission case takes up precious space that could be used for other transmission, engine and/or vehicle components. Since these areas are generally crowded with such components, access to the filter and strainers can sometimes be difficult. Accordingly, it would also be desirable to minimize the space allocated to these filter strainer components adjacent the transmission case as well as provide a combination filter/strainer design that would allow easy access for changing and/or cleaning of the filter and/or strainer elements.
Another problem sometimes experienced with the location of the filter and strainer in these hydraulic systems relates to their location relative to the hydraulic reservoir. When they are not located so as to assure a constant supply of hydraulic fluid, air can be drawn into the hydraulic lines and cause the pump to cavitate. Therefore, it would be desirable to place the filter/strainer assembly within the hydraulic reservoir to assure that a constant supply of hydraulic fluid is present in the lines of the system.